Consider that a printed circuit board having n conductor layers and n or (n−1) insulating layers laminated alternately so that the first layer from the surface of the printed circuit board is a conductor layer is perforated with a pulsed laser beam from a UV laser (hereinafter referred to as “laser beam” simply) in the background art. In this case, when the diameter of a hole to be processed is not smaller than 50 μm, the printed circuit board is irradiated a required number of times with top-hat beam pulses having a diameter substantially equal to the entrance diameter of the hole and having an energy distribution substantially uniform in a plane perpendicular to the optical axis of the beam or Gaussian beam pulses having an energy distribution like a Gaussian curve in a plane perpendicular to the optical axis of the beam (hereinafter referred to as “punching method”). Alternatively, the printed circuit board is irradiated with a top-hat beam or a Gaussian beam whose diameter is smaller than the entrance diameter of the hole and not larger than 50 μm by positioning the top-hat beam or the Gaussian beam at locations, for example, along a circumferential path, and the irradiation is repeated along some other circumferential paths (hereinafter referred to as “trepanning method”).
An optical scanner and a focusing lens are used to position The laser beam to a portion to be processed. A scan area defined by the size of the focusing lens is smaller than the printed circuit board. Therefore, when processing in the scan area is finished, the printed circuit board is moved to a next scan area horizontally relatively to the focusing lens so as to perform processing in the next scan area.
For example, JP-T-10-508798 (PCT/US95/08473) discloses a technique for perforating a material made of metal, insulator, etc. using ultraviolet light.
As an apparatus for performing laser processing while monitoring the condition of the processing, JP-A-10-85976 or JP-A-2001-102720 discloses a laser processing apparatus in which a beam splitter such as a polarizing beam splitter or a dichroic mirror is inserted into an optical path of a laser beam immediately after a laser oscillator or immediately in front of a work so as to deflect light reflected or emitted by the work from the optical path of the laser beam and sense the deflected light, or a laser processing apparatus in which a sensor is placed out of a laser optical path and near a work so as to sense light diffused or emitted by the work.
However, there is a large difference between a threshold value of decomposition energy of a conductor layer and a threshold value of decomposition energy of resin forming an insulating layer. Due to this difference, when one and the same portion is continuously irradiated with a pulsed laser beam so as to make a hole in a printed circuit board whose surface layer is a conductor layer, the diameter of the hole formed in an insulating layer under the conductor layer becomes larger than the diameter of the hole formed in the conductor layer. Thus, the conductor layer may overhang the hole formed in the insulating layer, or the hole formed in the insulating layer has a barrel-like shape such that the intermediate portion of the hole has a larger diameter than the entrance diameter.
When high-speed electroplating or filled plating (a plating method for filling a hole with growing a metal layer from the hole bottom) is performed on the hole such that the conductor layer overhangs the hole formed in the insulating layer, or the hole formed in the insulating layer has a barrel-like shape, the plating is concentrated in the hole entrance of the conductor layer. Thus, hollow portions (voids) are apt to occur in the plated layer. It is therefore impossible to shorten the plating time, and the processing efficiency cannot be improved.
Assume that processing is performed with energy density suitable to the conductor layer. In this case, when there is a variation in the thickness of the insulating layer, the surface of the conductor layer at the hole bottom may be damaged. Further, the conductor layer at the hole bottom may be pierced.
When the insulating layer is made of a so-called glass-containing base material which contains glass fibers, glass fibers may project largely inside a hole formed by selectively removing the resin of the hole side wall. A slight gap may be formed between the hole and a hole adjacent thereto because a larger quantity of resin than necessary is removed. Thus, plating may lead to electric connection between the adjacent holes. For this reason, the hole-to-hole pitch cannot be narrowed and so-called board densification contributing to reduction of the printed circuit board size cannot be achieved.